1. Field of the Invention
The present invention generally relates to a gate in a semiconductor device and a method for forming the same. More particularly, the present invention relates to a gate in a semiconductor device, which prevents a punch-through phenomenon without a C-halo pocket structure, and a method for forming the same.
2. Description of the Related Art
As well known in the art, as the size of devices is reduced by technology of forming a high integration DRAM of a small gate pitch, the short channel margin is likely to be deteriorated. In order to solve the problem, three-dimensional type transistors have been developed, one being a step gate asymmetry recess cell (which is referred to as “STAR cell”).
Generally, the STAR cell has a gate structure in which a vertical step is formed in a silicon substrate below a gate to increase the effective channel length, thereby enhancing the short channel margin. More specifically, as shown in FIG. 1, the STAR cell has a bit-line junction A located higher than a storage node junction B, thereby increasing the effective channel length X.
However, although the STAR cell has an increased effective channel length, a spacing X1 between the bit-line junction A and the storage node junction B is not changed in a deep region of the silicon substrate, and thus the STAR cell suffers from a punch-through phenomenon in the deep region of the silicon substrate.
In order to solve this problem, a C-halo pocket structure has been suggested, which can prevent the bit-line junction A and the storage node junction B from spreading to each other in a horizontal direction without affecting the doping concentration of a channel region determining the threshold voltage.
As shown in FIG. 2, in the C-halo structure, a diffusion region Y having a higher impurity concentration than that of a surrounding well region is formed by implanting impurity ions having opposite polarities into a portion near the bit-line junction A and the storage node junction B, thereby preventing the bit-line junction A and the storage node junction B from spreading to each other in a horizontal direction.
However, the gate of the C-halo pocket structure has a problem in that leakage current is increased due to damage of the gate during implantation of the impurity ions.
Accordingly, it is necessary to provide a method for forming a STAR cell which can solve the problem of punch-through without using the C-halo pocket structure.